What is the relation between double pointer and 2d array?

Question

void displayMatrix(int **ptr)
{
  printf("%d %d \n",**ptr,*(*ptr+1));
  *ptr++;
  printf("%d %d \n",**ptr,*(*ptr+1));
}

Is it a correct way to pass 2x2 array to function ?

displayMAtrix(array);

If you don't tell us what the type of `array` is, we can't answer this question. And please stop using `printf()` in C++. Use the iostreams library. — André Caron, Jul 20 '11 at 14:39
@Andre Actually I've homework is to add two Matrix but it was not working. actually i've problem with passing array to function. — Furqan, Jul 20 '11 at 14:41
@Tomalak yea, you're right it is not compiling.i think my concept about double pointer is wrong — Furqan, Jul 20 '11 at 14:46
This may help: http://stackoverflow.com/questions/4810664/how-do-i-use-arrays-in-c — Cubbi, Jul 20 '11 at 14:47
@furqan: A pointer to the first element of an array of pointers is not the same as a pointer to the first element of an array of arrays. :) _If someone told you that "arrays are pointers", **they were lying**._ — Lightness Races in Orbit, Jul 20 '11 at 14:48
@Tomalak yea my teacher told me, "arrays are constant pointers" — Furqan, Jul 20 '11 at 14:50
@furqan: I'm sorry to hear that. I suggest ignoring most of what (s)he says, and [reading a good book instead](http://jcatki.no-ip.org/fncpp/Resources). — Lightness Races in Orbit, Jul 20 '11 at 14:52
@furqan, you should take a look at c-faq.com/aryptr/index.html once it up. All your questions are answered there. — unkulunkulu, Jul 20 '11 at 14:53
@furqan to correct it you should say that arrays _decay_ into pointers in expressions. It's the way C-faq deals with it. Also the linked SO faq article is very good. — unkulunkulu, Jul 20 '11 at 14:54

score 3 · Accepted Answer · answered Jul 20 '11 at 14:51

If you have a 2D array with automatic storage duration (e.g. int matrix[2][2];), then no, this is not the correct way to pass a 2D array.

int f ( int, char ** )
{
  int matrix[2][2];
  displayMatrix(matrix);
}

To be standard compliant (there are other ways to make it work, but this is the standard, recommended way), you need to declare displayMatrix() as:

void displayMatrix ( int matrix[][2] );

You must declare the size of each dimension (possibly excluding the first). The reason behind this lies with the way 2D arrays are stored in memory. Wikipedia has a decent article on row-major order explaining the layout.

Alternate storage type

If you're allocating large matrices (e.g. for storing images), you'll usually end up with double pointers because you'll be allocating the memory differently. In this case, you usually have a 1D array of pointers with each item storing the pointer to a 1D array representing rows (or columns).

In that case, you would get something like:

// this function is over-simplified.  it may leak memory if any
// but the first `new` throws `std::bad_alloc`.
int ** new_matrix ( int m, int n )
{
  int ** matrix = new int*[m];
  for (int i = 1; (i < m); ++i ) {
    matrix[i] = new int[n];
  }
  return (matrix);
}

void displayMatrix ( int ** matrix, int m, int n );

int main ( int, char ** )
{
  int **const matrix = new_matrix(2, 2);
  displayMatrix(matrix, 2, 2);
}

score 0 · Answer 2 · edited May 23 '17 at 12:00

This is one of the reasons multidimensional raw arrays are so awkward in C++. If you know at least the last dimension of the array, you can do it this way:

void displayMatrix(int array[][2])
{
    // code goes here
}

The int** approach only works for dynamically sized arrays that were declared that way. It's not interchangeable with a int[2][2] array. For more info on the right way (i.e., when you're not doing a homework assignment) to do this, you can look at this SO question.

score 0 · Answer 3 · answered Jul 20 '11 at 14:53

0

I would do it like this:

struct Array2d {
   float *array;
   int size_x;
   int size_y;
};
void myfunction(Array2d &a) { a.array[x+y*a.size_x] = 10.0; }

answered Jul 20 '11 at 14:53

tp1

1,197
10
17

score 0 · Answer 4 · answered Jul 20 '11 at 14:54

Use a vector if you can help it

void displayMatrix(const std::vector<int> &dat);

If that fails, don't use arrays of arrays if you can help it. Calculate the offsets yourself.

void displayMatrix(int *dat);

If you intend to legitimately pass arrays of arrays, you have to specify the size of the arrays except for the largest dimension. It seems others have answered this before I could type it out.

What is the relation between double pointer and 2d array?

4 Answers4

Alternate storage type